Field Discovery and Evaluation of Native Spontaneous Plants for Soil Heavy Metal Pollution and Sustainable Phytoremediation Potential for Mining Wastelands

Abstract

Heavy metal pollution in mining wastelands poses a serious threat to soil quality and ecosystem sustainability, particularly in cold-climate regions where phytoremediation efficiency remains poorly understood. The present study aims to determine the risk of heavy metals in soils and the phytoremediation potential of native dominant spontaneous plants in lead–zinc mining wasteland and located in a cold region characterized by harsh winters and heavy snowfall. Soil samples (n = 60) and plant tissues (n = 84) were collected across the study area, and the concentrations of Cd, Pb, Zn, and Cu in rhizosphere soils and plant tissues were determined using atomic absorption spectrophotometry. Bioconcentration and translocation factors were calculated to evaluate plant metal enrichment and transport capacities. The results revealed that the concentrations of Cd, Pb, Zn, and Cu were at a relatively high potential ecological hazard level in the tailing ponds and surrounding areas. Field surveys showed that indigenous dominant spontaneous plants were better adapted to the harsh climatic conditions and poor soil matters than non-native plants, making them more economical and reliable candidates for phytoremediation. The study unexpectedly identified Commelina communis as a Cu phytostabilization candidate and found several metal-enriching plant species (n = 6), including Scirpus, Typha, Carex, Artemisia, Commelina, and Polygonum. The results can serve as a basic plant resource database for government institutions related to natural, ecological, and environmental sustainable management, offering new insights into self-sustaining phytoremediation strategies and sustainable ecological restoration in cold-region mining areas.

1. Introduction

The world is rich in lead-zinc mineral resources, with lead-zinc reserves amounting to approximately 3 × 10⁸ tons [1]. Lead-zinc mines are primarily distributed in China, Australia, Canada, Russia, Peru, Mexico, North Korea, the USA, India, and Iran. China ranks first in the world in both Pb-Zn mining and smelting capacity, making it a significant global leader in lead-zinc production. However, heavy metal (HM) pollution of soil in industrial wastelands is constantly increasing, and for years has attracted world attention. According to the National Soil Pollution Status Survey Bulletin, released on 17 April 2014, the overall soil contamination rate in China was 16.1%, with approximately 34.9% of the 775 industrial wasteland soil sampling sites exceeding contamination thresholds. Currently, China has about 8000 state-owned mining enterprises and approximately 230,000 collectively owned mines. During mining and smelting activities, various HMs are exposed to the surface and enter the surrounding soil environment through pathways such as slag discharge, wastewater effluent, tailings accumulation, atmospheric deposition, and surface runoff. Over time, these HMs accumulate, migrate, and spread, resulting in soil pollution in mining areas. HM pollution is significantly problematic due to its persistence, long-term impacts, and hidden nature. Even in industrial wastelands that have been abandoned for many years, the harmful impacts of these HMs on human health and the environment may still be present, as certain HMs exhibit high levels of toxicity even at low concentrations (e.g., Pb, As, Cd, Hg, etc.) [2]. Among them, Lead is regarded as one of the most permanent soil HM because it can stay in soils for over five thousand years with an above average biological half-life.

In response, the Ministry of Ecology and Environment of China issued the Guidelines for Promoting Soil Pollution Risk Management and Green, Low-Carbon Remediation in 2023, emphasizing green and low-carbon remediation design. Following the principle of “one site, one strategy,” the guidelines stress the importance of scientifically and rationally selecting risk management or remediation methods. Priority is encouraged for in situ remediation, bioremediation, and natural restoration as the primary techniques for risk management and remediation [3]. This green, low-carbon approach aims to enhance resilience to extreme climate events and disasters, improving adaptability to climate change.

Indeed, faced with the economic, environmental, and social developmental conditions in Northeast China, phytoremediation technology is regarded as the most practical and sustainable solution for restoring abandoned mines and addressing HM contamination of the soil. Compared to physical and chemical remediation methods, phytoremediation has distinct advantages, including lower costs, minimal environmental disturbance, and the ability to remediate soil and groundwater in situ without causing pollution transfer or secondary contamination [4,5,6,7,8,9,10].

Nevertheless, researchers have found that this green technology may have some obvious limitations, such as harmful concentrations of contaminants, toxic reactions, bioavailability, plant species, stress resistance, and so on. Furthermore, different climate condition, soil characteristics, lighting and temperature conditions, and moisture characteristics can result in various of plant species, as plants tend to seek the most suitable conditions for survival [11]. Therefore, identifying native, spontaneous, and dominant plants with hyperaccumulation potential and improving the efficiency of phytoremediation remained challenging yet critically important issues in the ecological restoration of lead-zinc mining regions [12,13,14,15,16,17,18,19,20].

The ultimate practical goal of the present study was to prioritize the planting of hyperaccumulative or HM-tolerant pioneer grasses that could rapidly cover the bare, abandoned mining land, forming a grass community during plant in situ restoration. This initial layer of vegetation could enhance the self-recovery ability of derelict land, improve soil nutrient conditions, and support the subsequent growth of shrubs and trees. This process could contribute to the establishment of a multi-layered plant community comprising trees, shrubs, and grasses, eventually forming a complete, functional, and sustainable mining ecosystem.

Unlike previous studies focusing on introducing species, our team emphasized the naturally regrown native dominanted plants that appeared and remained in the highly contaminated soils [21,22,23,24,25,26,27]. The identification of such spontaneous plants was significantly harder because some species were observed in extremely low abundance, just for one or two individuals, which showed their extraordinary environmental adaptability and remediation potential as candidates for sustainable phytoremediation.

Therefore, it was hypothesized that hyperaccumulator species for HMs could be identified in the study. To test this hypothesis, in the early stage of the research process, a large-scale investigation of soil HM pollution was conducted across the entire mining area to determine the extent of pollution and to support the following evaluation and analysis. In the mid-stage, the research scope was narrowed to areas undergoing natural in situ remediation with wild plants growing naturally, where the HM content in the plants and rhizosphere soil was tested to assess the plants’ tolerance to HM pollution and their remediation potential. In the later stage, plant species with HM tolerance or hyperaccumulation traits were screened from the naturally occurring vegetation, and appropriate planting configurations were determined in preparation for broader application.

2. Materials and Methods

2.1. Study Site

The study area is located across higher latitudes of the northern hemisphere, defined by weather characteristics such as lengthy, cold, dark, snowy, or slushy winters. It was generally considered that this frigid climate poses significant challenges to vegetation growth. Moreover, it was more difficult to clear heavy metal pollution in mining areas through traditional chemical and physical tragedies than bioremediation under the low temperature, freeze-thaw, or frozen conditions of the Northeastern regions. Especially, low temperature affected the optimal range for their metabolism and normal physiological activities of microorganisms, and even influenced soil pollutant degradation and heavy metal transfer and stabilization [28]. The soil in abandoned lead-zinc mining areas was severely contaminated with HM and other mineral processing reagents; its nutrient content (such as nitrogen, phosphorus, and potassium, as well as organic matter) was remarkably deficient, it had poor structure, and its soil pH was extreme, all of which compounded the negative effect on plant life [29].

The present study was conducted in the surrounding areas of the QZ lead-zinc Mine, located at the foot of FS Mountain, LN Province, Northeastern China, near the border with North Korea. The mountain is mostly covered with snow in winter. Here, the main nonferrous metal mineral resources include lead, zinc, copper, and gold. QZ lead-zinc Mine was originally a leading, state-owned mining enterprise and the first large-scale lead-zinc mine in Northeast China. The mining area comprises over ten middle- and small-sized deposits, such as ZG, DN, NS, MP, BS, and XG, with a total area of approximately 50 km². The ore deposits are located in the central part of the LD ancient rift valley, and are a typical example of such mining fields in China. The geographic coordinates are longitude 123°34′ E and latitude 40°43′ N in the winter region. The study area is presented in Figure 1.

The studied area features a temperate continental monsoon climate with an average annual temperature of 5.5 °C, an annual precipitation of no more than 1000 mm and a frost-free period of approximately 140 days. The summer is quite short and July is the hottest and wettest month of the year, with an average temperature between 15 and 24 °C. The winter season is cold because of frequent and heavy snow with an average temperature ranging from −7 to 2 °C [30]. The dominant type of soil is mountain brown earth, and corn is the main cultivated crop. The region is abundant in fruits such as strawberries, blueberries, peaches, and hardy kiwifruit (Actinidia arguta), thriving under the favorable climate and fertile soil conditions.

Figure 1. Geological sketch map and general study area of the QZ Lead–zinc Mine (modified after Ma et al., 2012) [31]. The red polygon indicates the general study area selected as a reference for soil and plant sampling. 1. Quaternary; 2. Schist of Gaixian Formation; 3. Marble of Dashiqiao Formation; 4. Plagioclase amphibolite schist of Langzishan Formation; 5. Mixed granulite Anshan Group; 6. Granite; 7. Migmatitic granite; 8. Diorite; 9. Syncline; 10. Inverted syncline; 11. Anticline; 12. Inverted anticline; 13. Fault; 14. Ore spot.

Figure 1. Geological sketch map and general study area of the QZ Lead–zinc Mine (modified after Ma et al., 2012) [31]. The red polygon indicates the general study area selected as a reference for soil and plant sampling. 1. Quaternary; 2. Schist of Gaixian Formation; 3. Marble of Dashiqiao Formation; 4. Plagioclase amphibolite schist of Langzishan Formation; 5. Mixed granulite Anshan Group; 6. Granite; 7. Migmatitic granite; 8. Diorite; 9. Syncline; 10. Inverted syncline; 11. Anticline; 12. Inverted anticline; 13. Fault; 14. Ore spot.

2.2. Soil and Plants Sampling

The soil and plant sampling was performed during four field campaigns from September 2021 to July 2022 within the growing season. Meanwhile, the four metallic trace elements (Cu, Zn, Pb, and Cd) were inspected from the study area to increase sample representativeness and ensure sufficient plant material for analysis, rather than to evaluate seasonal variation in metal uptake. So, samples collected at different times were used for subsequent analysis. In particular, sampling points (e.g., “No.1 land,” “No.2 land”) were purposely selected due to the presence of native spontaneous plants, as the study aimed to assess the phytoremediation potential of plants capable of surviving under heavy metal stress (Figure 2). Plant community surveys were conducted in long-abandoned mining wastelands, where these plants had developed naturally without any human intervention. Representative, healthy, and dominant plant species were identified in the field. The aboveground stems and leaves were separated from the underground roots of multiple individuals of each species, then labeled, and stored in differently sized poly bags. To complete the research and experiments, those fresh rhizosphere topsoil samples (0–20 cm depth) were taken from the surface wastelands with a wooden spatula by using an S-shaped pattern to account for small-scale spatial variability. These subsamples were then transported to the laboratory for processing and placed in clean, labeled bags before being thoroughly mixed to obtain one composite soil sample (~1 kg) by the quartering method. In total, 60 soil samples and 84 plant samples were obtained across multiple sites and species. In the laboratory, the soil samples were air-dried, ground, sieved, homogenized, and stored in clean containers prior to chemical analysis [32]. Accordingly, the sampling design emphasizes functional representativeness relevant to ecological restoration objectives, rather than statistical representativeness of site-wide contamination.

2.3. Chemical Analysis

The soil samples were cleaned to remove litter, fallen leaves, and visible plant roots, then air-dried, and sieved through a 200-mesh sieve for preparation. These samples were ground using an agate mortar and pestle, which was thoroughly cleaned between samples to prevent cross-pollution. The concentrations of HMs in the soil samples were determined using atomic absorption spectrophotometry after digestion with a mixed acid system of HNO₃, HF, and HClO₄ (mass ratio 4:4:2), following standard procedures [33,34].

The plant samples were thoroughly washed with tap water to remove surface-adhered soil particles, and rinsed with deionized water. These cleaned samples were deactivated at 105 °C for 45 min, dried at 70 °C for 72 h to constant weight, ground, sieved through a 200-mesh sieve, and digested with a mixture of HNO₃ and HClO₄ (mass ratio 8:2). The HM concentrations (Cd, Pb, Zn, and Cu) were then measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) [35,36].

Quality assurance and quality control procedures were implemented throughout the analytical process. Procedural blanks were designed to examine potential pollution in digestion and analysis. Certified reference materials for soil and plant matrices were analyzed to measure analytical accuracy. Spike recovery tests were implemented, and recoveries for all target HMs were within acceptable analytical ranges. Method detection limits were identified based on replicate blank measurements and were all below the measured concentrations in all samples. All chemical analyses were completed in triplicate as analytical replicates, and the reported HM concentrations represent mean values of replicate measurements.

2.4. Data Analysis

2.4.1. Soil Data Analysis Methods

Currently, many assessment methods are used to determine the degree of soil contamination by using the common geochemical background. This study focused on the index of geoaccumulation (Igeo) proposed by Muller and the potential ecological risk index (PERI) developed by Hakanson [37,38]. Different evaluation methods were used depending on the environmental conditions and research objectives.

Index of Geoaccumulation (Igeo):

From the perspective of environmental geochemistry, evaluating HM pollution of soil requires not only considering anthropogenic pollution factors and environmental geochemical background values but also accounting for factors that might cause variations in background values due to natural diagenetic processes [37].

I_geo = log₂[C_i/(kB_i)]

(1)

C_n: Concentration of element n in soil;

B_n: Geochemical background value of element n in soil;

k: A coefficient (generally set to 1.5) used to account for potential variations in the background value due to regional differences. It represents sedimentary characteristics, lithological geology, and other influencing factors.

Potential Ecological Risk Index (PERI):

From a sedimentological perspective, evaluating HM pollution in soil or sediment involves not only considering the concentrations of HM but also including their ecological effects, environmental impacts, and toxicological properties. This approach reflects not only the impact of various pollutants in soil or sediment under specific environmental conditions but also the combined effects of multiple pollutants in the environment. Accordingly, based on the contamination factor (C_f) and the corresponding toxicity response factor (T_r), the potential ecological risk factor (E_r) for each HM and the comprehensive ecological risk index (RI) were calculated following Formula (2) [38].

$$C_f^i=C{s}^i/C_n^i; C_d={\displaystyle \sum _{i=1}^m{C}_f^i}; E_r^i=T_r^i\cdot C_f^i; RI={\displaystyle \sum _{i=1}^m{E}_r^i}={\displaystyle \sum _{i=1}^m{T}_r^i\cdot C_f^i}$$

(2)

$C_f^i$: contamination factor of a specific HM relative to the environmental background value;

$C{s}^i$: concentration of HM;

$C_n^i$: background value of the HM;

$C_d$: integrated pollution level of HM;

$E_r^i$: potential ecological risk factor of a single HM;

$T_r^i$: toxic response factor of a single HM;

RI: potential ecological risk index of multiple HM.

2.4.2. Plant Data Analysis Methods

Hyperaccumulators have a key role in phytoremediation according to their ability to tolerate and accumulate high levels of metals [39]. Such plants must simultaneously meet five criteria: Toxicity tolerance: they must tolerate high concentrations of HM that would otherwise poison or kill ordinary plants [40,41]. Enrichment capacity: the concentration of HM in the aerial parts of the plant must exceed that in the soil (BCF > 1). Translocation capacity: the concentration of HM in the aerial parts must exceed that in the roots (TF > 1) [42,43]. Lowest criticality: the HM content in the aerial parts (on a dry weight basis) must reach critical levels: Cu ≥ 1000 mg/kg, Zn ≥ 10,000 mg/kg, Pb ≥ 1000 mg/kg, and Cd ≥ 100 mg/kg, or be 10–100 times higher than in uncontaminated ordinary plants [44,45]. Agronomic advantages: the plants must exhibit fast growth rates, short life cycles, high aboveground biomass, and the ability to simultaneously accumulate two or more HM.

The tolerance and absorptive capacity of hyperaccumulators to HM are fundamental characteristics for extracting and remediating contaminated soils. When soils contain multiple HM, plants must not only exhibit significant tolerance but also possess a strong ability to absorb these HM. Consequently, the bioconcentration factor (BCF) and translocation factor (TF) are critical indicators for assessing a plant’s capacity to absorb and transfer HM [46]. BCF is a numerical value that represents the ability of HM-tolerant plants to remove HM from the polluted soil. TF refers to the measurement value indicating the ability of HM pollutants from the plant roots to the aboveground parts [47,48,49,50,51,52,53].

Bioconcentration Factor (BCF):

BCF = C_plant/C_soil

(3)

where C_plant represents the concentration of the HM in the plant (either in roots or shoots), and C_soil is the concentration of the HM in the rhizosphere soil.

Translocation Factor (TF):

TF = C_shoot/C_root

(4)

where C_shoot and C_root represent the concentrations of the HM in the plant’s shoots and roots, respectively.

Statistical analysis of variables and generation of figures was performed using the RStudio software, version 4.5.1 [54,55].

Given the high spatial heterogeneity of tailings environments and the uneven distribution of native spontaneous vegetation, the statistical analysis in the present study was limited to descriptive statistics to characterize HM concentrations and phytoremediation indices, rather than inferential statistical comparisons among sites or species.

3. Results and Discussion

3.1. Evaluation of Soil HM Pollution Across the Entire Mining Site

In order to comprehensively assess the soil pollution status, the HM contamination survey was conducted across the entire Pb-Zn mining site. Based on the previous laboratory analysis of soil samples collected from the mining area by the authors [31], the original individual replicate values were not available for reanalysis. However, all HM concentrations were obtained from direct field sampling and laboratory measurements, and the data provided convincing evidence of spatial distribution and levels of heavy metal pollution (Figure 3).

In the study area, three groups of mixed samples were collected from waste dump, tailing pond, and surrounding farmland of the Pb-Zn mining area and analyzed for HM, such as Cu, Zn, Pb, Cr, Cd, Hg, and As. The results clearly showed that HM concentrations highly exceed background values, particularly in tailing ponds, indicating that mining activities have introduced HMs into soils of Pb-Zn mining areas and even surrounding farmlands.

The index of geoaccumulation provided a straightforward classification of HM pollution levels and clearly reflected the degree of HM enrichment. However, it focused on individual metals without considering bioavailability, relative contribution proportions, or geographical spatial variations. On the other hand, the potential ecological risk index addressed these shortcomings by comprehensively reflecting the potential impact of HM on the ecological environment. However, its toxicity factors involved an element of subjectivity. Therefore, this study evaluated soil HM pollution in the QZ Lead–zinc Mine area by integrating and drawing on the complementary strengths of Igeo and PERI.

The index of geoaccumulation is divided into seven levels, ranging from 0 to 6, representing pollution levels from “unpolluted” to “extremely strong pollution” (Figure 4). At the highest level (Grade 6), the concentration of HM elements is hundreds of times higher than the background value. The following table presents the classification of polluted soils according to Igeo by Muller [37].

HM pollution levels in tailing ponds were summarized as follows (Figure 4):

Cr: slightly polluted;

Hg: slightly to moderately polluted;

As: moderately polluted;

Cu: strongly polluted;

Cd, Pb, Zn: extremely polluted.

The pollution levels in the study area were summarized as follows:

Dump sites and surrounding farmlands: Grade 3 (moderately to strongly polluted);

Tailing ponds: Grade 6 (extremely polluted).

Furthermore, using the standardized HM toxic response coefficients developed by Hakanson as the basis for evaluation, the potential ecological risk level of soil HM was determined according to the relationship between PERI and its classification [38].

The toxic response coefficients for the HMs were as follows: Cu = Pb = 5; Zn = 1; Cr = 2; Cd = 30; Hg = 40; As = 10.

The toxicity response coefficients prescribed by Hakanson, and Formula (2). Figure 5 revealed the following about HM pollution levels in tailing ponds:

Cr was at a slight potential ecological hazard level.

As was at a moderate potential ecological hazard level.

Cu, Zn, and Hg were at a relatively high potential ecological hazard level.

Pb was at a severe potential ecological hazard level.

Cd was at an extremely severe potential ecological hazard level.

From the perspective of the overall PERI, the waste dumps and surrounding farmlands were heavily polluted, while the tailing ponds were severely polluted.

The potential ecological hazard levels of soil HMs in the study area, ranked from high to low, were as follows (Figure 4):

Waste dumps: Pb → Cd → As → Cu → Hg → Zn → Cr

Tailing ponds: Cd → Pb → Hg → Zn → Cu → As → Cr

Surrounding farmland: Hg → Cd → Pb → As → Cu → Zn → Cr

From the perspective of the potential ecological hazard coefficient (Eri) of individual heavy metals, soil HM pollution was primarily dominated by Cd and Pb, with Cr being the least significant. Regarding the potential ecological risk index (RI) of multiple HM, the surrounding farmland exhibited relatively mild ecological hazards, whereas the tailing ponds showed extremely severe ecological risks. The ranking of ecological hazard levels was tailing ponds, waste dumps, and surrounding farmland.

By comparing the results of the PERI and Igeo methods, it was evident that the evaluation results were largely consistent. This indicated that the combined use of these two methods was both complementary and mutually corroborative.

Comprehensive analysis suggested that soil HM pollution in the mining area was severe, with the primary pollutants (Cd and Pb), accompanied by other HM such as Zn and Cu. In order to mitigate soil HM pollution, the priority should be to control the contamination caused by Cd and Pb. Additionally, due to the significant interaction between Cd and Zn, it was also crucial to strengthen the control of Zn pollution. So as for vegetation restoration, it was essential to select plant species with strong overall resistance to Cd, Pb, Cu, and Zn, particularly those with high accumulation capacities for Cd and Pb.

3.2. HM Content Analysis in Rhizosphere Soil of Plants in Tailing Ponds and Surrounding Areas

After determining the soil pollution status of the entire mining area, the HM pollution assessment of the rhizosphere soil was conducted to narrow the research scope and further explore the relationship between the soil and plants, as well as the transformation patterns of HM between the plants and soil. Therefore, according to previous experimental results, Cd, Pb, Zn, and Cu, the four major pollutants, were selected as the target HMs. Given that the highest HM concentrations occurred in the tailing ponds, HM pollution in the tailing ponds and the surrounding areas was the primary focus of the study. Accordingly, a total of 60 soil samples were collected from the study area. Among them, all HM concentrations were substantially higher than the local soil background, with Pb and Zn showing the highest values in the corn field next to the tailing ponds (Figure 6).

Furthermore, pH ranged from 7.65 to 8.61, which implied that the soil in the study area was alkaline. The alkaline soil environmental conditions might affect HM speciation and bioavailability. For example, Pb and Cd were likely to form less soluble complexes, which reduced their mobility in soils and supported retention in the rhizosphere. This might partly explain the pronounced accumulation of lead in roots and the extremely limited translocation to aboveground parts observed in several dominant species, particularly Typha orientalis.

3.3. Analysis of Dominant Plant Species and Their Biological Characteristics in Tailing Ponds and Surrounding Areas

The mine is primarily composed of lead–zinc ore, followed by copper ore. Cadmium, lacking an independent ore deposit, is often associated with lead–zinc ore. Therefore, the plant potential of absorbing HM in the Pb-Zn mining area depends on four major HM, such as Cu, Zn, Pb, and Cd.

A total of 27 dominant native plant species, such as Chenopodium album, Kochia scoparia, Cassia nomame, Sonchus arvensis, Polygonum orientale, Amorpha fruticosa, Pueraria lobata, Robinia pseudoacacia, Carex, Artemisia gmelinii, Rubus crataegifolius, Chenopodium glaucum, Amaranthus tricolor, Beckmannia syzigachne, Commelina communis, Calystegia japonica, Metaplexis japonica, Zea mays, Plantago major, Scirpus validus, Scirpus radicans, Artemisia gmelinii Waldst., Typha orientalis, Salix matsudana, Juncellus serotinus, Bidens biternata, and Oenothera biennis, were discovered from the tailing ponds area after field investigations and expert identification by authors. These species belonged to the plant families such as Chenopodiaceae, Asteraceae, Rosaceae, Fabaceae, Amaranthaceae, Poaceae, Leguminosae, Commelinaceae, Cyperaceae, Apocynaceae (Metaplexis japonica), Polygonaceae, Typhaceae (Typha orientalis), and Salicaceae (Salix). Among them, Asteraceae, Rosaceae, and Poaceae accounted for a significant proportion, due to their strong seed dispersal abilities and broad adaptability. Additionally, Fabaceae species provide the added benefit of nitrogen fixation.

Poaceae is particularly notable as the most economically important family among seed plants. It serves as a primary food source of human and livestock, and provides raw materials for various industries, including starch production, sugar processing, brewing, paper-making, weaving, and construction.

In terms of life forms, the survey identified 11 annual herbaceous plants and eight perennial herbaceous plants, comprising 70.4% of the collected species. For example, herbaceous plants showed considerable advantages, which had powerful HM tolerance and resistance to infertility, drought, and waterlogging. These native grasses are highly adapted to the local climatic and environmental conditions and thrive even in heavily polluted areas near the tailing ponds, which can be regarded as pioneer species for ecological restoration.

On the hillsides surrounding the tailing ponds, those species such as Artemisia gmelinii, Robinia pseudoacacia, Quercus mongolica, Rubus crataegus, Pueraria lobata, and grasses of the Poaceae family were observed growing robustly in clustered communities. Vegetation was sparse in flooded areas of the tailing ponds, but Commelina communis, a Cu phytostabilization candidate species, and Bidens biternata, which exhibited relatively higher Cu accumulation potential, both displayed better growth performance in wetland environments [56,57,58].

Additionally, a variety of aquatic and water-resistant plant species, like Scirpus validus, Scirpus radicans, Typha orientalis, and Pueraria lobata, were found to thrive with substantial biomass. These species possess abundant root systems capable of adsorbing HM pollutants from water, thereby mitigating water contamination. Currently, such plants are widely utilized, primarily in artificial systems and biological pond engineering.

3.4. Analysis of HM Content in Dominant Plants

In terms of the field survey and laboratory analysis results, plant species with robust resistance to the toxic effects of HM, particularly Cd, Pb, Zn, and Cu, could be regarded as super candidates. Therefore, the study of HM accumulation in those dominant plants mainly focused on Cd, Pb, Zn, and Cu. The top ten plant species had consistently high accumulation performance across both roots and shoots, which suggested their strong potential for phytoremediation in tailing wasteland. In the rankings, the first one was Typha orientalis, followed by Commelina communis, Amaranthus tricolor, Scirpus radicans, Beckmannia syzigachne, Polygonum orientale, Juncellus serotinus, Salix matsudana, Bidens biternata, and Plantago major (Figure 7).

As seen from Figure 8, among the dominant plant species, Typha orientalis, Commelina communis, and Amaranthus tricolor exhibited the most powerful HM accumulation potential. Furthermore, Typha orientalis also displayed significant tolerance to HM, especially Pb and Zn. Moreover, the shoot of Amaranthus tricolor showed the best tolerance to Pb, and the shoot of Polygonum orientale had the highest sorption capacity to Cu (Figure 8). The typical HM concentrations in general plants were as follows: Pb 0.1–41.7 mg/kg; Zn 1–160 mg/kg; Cd 0.2–3 mg/kg; Cu 0.4–45.8 mg/kg. Among the dominant plants mentioned above, many exhibited much higher concentrations of Cd, Pb, Zn, and Cu that far exceeded the upper limits of normal values, indicating their strong adaptability to the polluted soil. These plants can thrive in areas with severe HM contamination and show considerable tolerance to HM. For instance, the Cd concentrations in Scirpus radicans and Typha orientalis reached as high as 13.9925 mg/kg and 12.1773 mg/kg, respectively. Similarly, the Zn concentrations in Typha orientalis and Commelina communis were 1083.7798 mg/kg and 738.9402 mg/kg, individually, while the Cu concentrations in Beckmannia syzigachne, Typha orientalis, and Polygonum orientale all exceeded 100 mg/kg.

As seen from Figure 9, in the roots, Typha orientalis had the highest accumulation abilities of HM, especially Cu, but in the shoots, Amaranthus tricolor had the highest. Additionally, the roots of these plants, such as Typha orientalis, Juncellus serotinus, and Beckmannia syzigachne, exhibited strong resistance to the combined pollution of Cu, Pb, and Zn, while Amaranthus tricolor and Commelina communis demonstrated better tolerance to Cu, Pb, and Zn, which makes them worthy of further investigation. For the HM of Cd, the values of the shoots were higher in Scirpus radicans, Salix matsudana, and Bidens biternata. On the contrary, the concentrations of Cu were extremely low in Cassia nomame, Oenothera biennis, and Salix matsudana (Figure 9).

3.5. Accumulation Ability of Dominant Plants to HM in Tailing Wasteland

Plant roots are crucial for absorbing water and mineral nutrients, and are the primary parts where plants experience HM stress in the soil. The capacity of plants to accumulate HM is quantified by the BCF, also known as the “biological absorption coefficient.” The value of BCF reflects the ability to absorb, accumulate, and amplify HM of plants, indicating the plant’s efficiency in enriching and absorbing these metals. The BCF also provides insight into the difficulty of HM migration within the soil–plant system, revealing the plant’s overall accumulation ability. A larger BCF indicates a stronger ability of plants to absorb and translocate HM from the soil into the plant, highlighting a greater enrichment capacity.

Because the biomass of the aboveground parts of plants is easier to obtain than the underground parts, a higher BCF of these parts is particularly beneficial for plant-based extraction and soil restoration. In a single soil pollution situation, the BCF of HM in plants typically decreases as the concentration of metals in the soil increases. Consequently, the BCF for plants in polluted soils is usually less than 1. This is another key reason why this study focuses on native dominant plants in the abandoned tailing wasteland, rather than relying solely on controlled laboratory conditions to measure the amount of HM absorbed by plants. After field investigation, these spontaneous native plants with higher BCF can be rapidly used in a phytoremediation project.

Based on the value of BCF, the following results could be achieved: Typha orientalis (Plot #3), Polygonum orientale (Plot #6), Scirpus radicans (Plot #3), Juncellus serotinus (Plot #3), Pueraria lobata (Plot #6), Commelina communis (Land #1), Beckmannia syzigachne (Plot #1), and Salix matsudana (Plot #3) all exhibited high BCF values for the four HMs (Cd, Pb, Zn, and Cu), indicating their strong ability to accumulate these metals. These plants were recommended as potential pioneer species for phytoremediation. However, Pueraria lobata has significant medicinal value and could be applied in medicinal fields after HM accumulation identification.

The degree of soil HM contamination varied across different research plots: #1 > #3 > #6. As a result, the most suitable plant species for ecological restoration would differ by location. Near the flooded area of the derelict tailing pond, Commelina communis and Beckmannia syzigachne could be used. For wetland restoration downstream of the tailings dam, Typha orientalis, Scirpus radicans, Juncellus serotinus, and Salix matsudana were suitable due to their high tolerance to water and humidity. On slopes around the tailing pond, Polygonum orientale and Pueraria lobata were more appropriate. These dominant plants not only exhibited strong HM enrichment abilities but also thrived in the current environmental conditions, demonstrating strong environmental adaptability. Therefore, they were of considerable scientific interest for further research and application.

Plant species exhibiting strong overall HM enrichment capacities, particularly for Pb, Zn, and Cu, included Carex, Artemisia gmelinii, Plantago major, and Juncellus serotinus, all of which showed BCF values greater than 1. Artemisia gmelinii, in particular, showed exceptional enrichment abilities for these three HM and warrants further attention. Previous studies have noted that Carex had a strong tolerance to Pb, Zn, Cu, and Cd, making it one of the most dominant plants in Cu-Zn mining wastelands [59,60]. Because Carix exhibited excellent adaptability to high concentrations of Cu (3610 mg/kg) and Zn (2570 mg/kg), it was a promising species for phytoremediation in Pb-Zn and Cu-Zn mine wastelands.

Additionally, specific plants demonstrated comparative advantages in accumulating individual HMs. In short, for the overall values of BCF, Typha orientalis was the highest, followed by Polygonum orientale, Scirpus radicans, and Commelina communis. Among them, Scirpus radicans showed the strongest ability to accumulate Cd, while Typha orientalis excelled at accumulating Zn. Polygonum orientale was the most efficient in Cu accumulation, while Typha orientalis performed best in Pb accumulation. Furthermore, Amaranthus tricolor, Beckmannia syzigachne, and Commelina communis displayed strong resistance to Zn and Cu, with BCF values exceeding 1. Notably, Commelina communis, an unexpectedly dominant species in this study, was a typical HM phytostabilization candidate, demonstrating a strong capacity for enriching both Pb, Zn, and Cu in its aboveground and underground systems—a characteristic that made it particularly noteworthy (Figure 10 and Figure 11).

Although Commelina communis has been previously reported as a copper hyperaccumulator in warmer regions [56,57,58], the present experimental results revealed a different HM accumulation and translocation potential in cold-climate Pb–Zn mining areas. In the present study, Commelina communis did not thoroughly meet the criteria of a classical hyperaccumulator. Instead, it exhibited BCF greater than 1 for Zn and Cu, whereas TF exceeded 1 for Pb and Zn, accompanied by high metal concentrations in both aboveground and underground parts. The results displayed that HM accumulation ability of Commelina communis is significantly context-dependent and influenced by climatic conditions. Indeed, Commelina communis, with high Zn accumulation and translocation potential, represents a promising candidate for phytoremediation in cold-climate Pb–Zn mining areas, where ideal hyperaccumulators are rarely encountered under field conditions.

The highest metal accumulation observed in Typha orientalis was related to Pb, which was mainly accumulated in its roots, as indicated by high BCF values and extremely low TF values. Such higher accumulation of HMs in roots is a kind of protection mechanism for plants in mining areas, which can make them avoid toxic hazards of HMs and survive in seriously polluted environments by immobilizing lead in roots [61]. This restricted translocation was likely associated with strong lead binding to root cell walls, apoplastic sequestration, and limited xylem loading, which together constrain lead mobility in the plant. As a result, Typha orientalis can tolerate high lead concentrations while minimizing ecological risks related to HM transfer. Given this behavior, Typha orientalis was widely used in phytoremediation programmes, aiming to stabilize contaminants and restore degraded and polluted wetlands.

Polygonum orientale also exhibited robust Pb and Cu accumulation abilities. The species is tolerant of both flooding and drought conditions with high vitality, with large stature, lush green foliage, and dense red flowers. It is resistant to pests and diseases, making it suitable for low-maintenance management. It has rapid growth, ornamental qualities, and strong adaptability, which make it an ideal candidate for phytoremediation in urban industrial wastelands.

3.6. Transfer Ability of Dominant Plants to HM

The transfer ability of plants to HM can be indirectly assessed by TF, which measures the movement of HM from roots to aboveground tissues of plants. A higher TF value indicates stronger tolerance to HM and greater ability to transfer and accumulate metals from roots to stems and leaves.

When the TF is greater than 1, it suggests that the plant absorbs or accumulates more HM in the aboveground parts (such as stems and leaves) over a shorter regeneration cycle, with only a small proportion remaining in the roots. Upon harvesting, most of the HM accumulated in the plant’s aboveground parts can be removed from the contaminated soil as soon as possible. As these plants are cultivated and harvested, the concentration of soil HM decreases, thereby achieving the goal of remediating the contaminated land.

Based on the comparison of TF, the following conclusions can be drawn:

Kochia scoparia (Plot #2), Artemisia gmelinii (Plot #1), Amaranthus tricolor (Land #1), Salix matsudana (Plot #3), Metaplexis japonica (Plot #8), Calystegia japonica (Land #8), Robinia pseudoacacia (Plot #6), and Chenopodium glaucum (Plot #1) all exhibited high TF for the HMs, Cd, Pb, Zn, and Cu, indicating that these plants have strong abilities to absorb and accumulate these metals in their aboveground tissues. Among them, Metaplexis japonica, Robinia pseudoacacia, and Chenopodium glaucum demonstrated particularly remarkable comprehensive accumulation abilities for all four HMs.

Furthermore, Robinia pseudoacacia stands out due to its strong adaptability, nitrogen-fixing root nodules, resistance to sulfur dioxide, chlorine, and photochemical smog, and its ability to absorb lead vapor. Therefore, Robinia pseudoacacia can be considered a valuable pioneer species for sand fixation, soil conservation, and greening in industrial or mining areas, barren hills, and wasteland rehabilitation. Studies indicated that hyperaccumulators were often herbaceous plants from families such as Cruciferae, Compositae, and Solanaceae, and some large woody plants like those in the Leguminosae family also showed super potential. Woody plants tend to possess large biomass, high metal enrichment capacity, and longer restoration cycles, making the identification of suitable leguminous woody species a key point of research for soil HM remediation.

The survey results showed that Artemisia gmelinii and Amaranthus tricolor were common plants in the abandoned mining lands, demonstrating good growth, strong adaptability, and efficient HM accumulation capabilities, particularly for Pb, Zn, and Cu. These plants were worthy of further application and promotion in restoration efforts. Salix matsudana, with strong abilities to accumulate Cd and Zn, also exhibited excellent translocation capabilities for these metals, making it an ideal species for ecological restoration in areas contaminated with Cd and Zn.

In summary, the overall values of TF in Kochia scoparia and Polygonum orientale were higher. Additionally, plants such as Kochia scoparia and Salix matsudana showed strong translocation abilities for Cd, Cu, and Zn, while Chenopodium album and Commelina communis excelled in transferring Pb and Zn. Sonchus arvensis and Artemisia gmelinii were particularly effective at transferring Zn and Cu. Overall, Kochia scoparia had the highest translocation ability for Cd, Artemisia gmelinii for Pb, Salix matsudana for Zn, and Oenothera biennis for Cu (Figure 9 and Figure 10).

The two key characteristics of hyperaccumulator plants are that both BCF and TF exceed 1. Although these plants might not entirely meet the critical standards for hyperaccumulator plants, they still exhibited strong HM tolerance. According to the combined values of bioconcentration and translocation factors, Artemisia gmelinii showed strong resistance to Zn and Cu, Salix matsudana to Cd and Zn, Amaranthus tricolor to Zn and Cu, Commelina communis and Scirpus radicans to Zn, Polygonum orientale and Typha orientalis to Cu, and Bidens biternata to Cd. These plants were suitable candidates for HM tolerance in ecological restoration projects of derelict lead–zinc mines.

Salix matsudana, a deciduous tree, excelled in transforming Zn. It is light-loving, cold-resistant, and can thrive in both wetlands and drylands. With a developed root system, it is highly resistant to wind, grows rapidly, and is easily propagated. Additionally, it has strong resistance to pests and air pollution. Its large biomass makes it useful for timber production, while its other characteristics, such as suitability for road-side planting, shelter forests, and desert afforestation, make it a valuable species for ecological restoration in derelict Pb-Zn mine areas.

Artemisia gmelinii, a semi-shrub with woody main roots, strong drought and cold resistance, and high seed production, was well-suited for field planting. It is capable of producing numerous tillers and new branches. Scirpus radicans, a kind of wetland plant, thrived in flooded areas and was commonly found on riverbanks and lakesides and in swamps, often growing in small groups or forming pure communities due to its strong tillering ability.

4. Conclusions

Under the climate, soil conditions, and the extent of HM pollution in QZ mining areas, native spontaneous plants are more suitable than non-native species for ecological restoration. Furthermore, several dominant native plants identified in this study exhibited strong tolerance to and accumulation of Cd, Pb, Zn, and Cu, which indicated their potential effectiveness in sustainable phytoremediation of Pb-Zn mining wastelands.

Based on the above-mentioned findings, future screening of HM-tolerant or accumulator plants should focus not only on individual species but also on dominant taxa at the genus level in nonferrous metal mining areas, which might improve the efficiency of species selection for large-scale restoration projects.

In addition, although most of the plant species identified can not qualify as perfect hyperaccumulators, they still played a significant role as HM-tolerant plants. Herbaceous species are suitable for early-stage restoration to achieve rapid vegetation cover, erosion control, and reduction of HM mobility, while woody species such as Salix matsudana may be introduced in later stages to enhance biomass accumulation and ecosystem sustainability.